1. Field of the Invention
The present invention relates to an image forming apparatus including an exposure device.
2. Description of the Related Art
An electrophotographic image forming apparatus (hereinafter, referred to as “image forming apparatus”) forms an image on a recording medium by using an electrophotographic process. Examples of the image forming apparatus include a laser beam printer and a copy machine.
In a digital image forming apparatus, an exposure device forms an electrostatic latent image on an image bearing member by exposing the image bearing member based on image data read by a charge coupled device (an image pickup device) of an image reading portion or image data input to a control portion as an electrical signal from an external device. An exposure device employing a semiconductor laser performs an exposure of a dot line based on image data in a main scanning direction on a rotating image bearing member that is uniformly charged. The exposure device then performs an exposure in a sub scanning direction by a rotation of the image bearing member. This makes an electrostatic latent image be formed on the image bearing member as a whole series of dots.
In the digital image forming apparatus, it is required to form an image including a line image area (text portion) and a solid image area (picture portion) with high quality.
For example, when forming an image including a line image area and a solid image area in a conventional technology, the line image area is exposed with the same laser light intensity as that for the solid image area. In this case, if a toner bearing amount (toner amount per unit area) that is determined to be appropriate with a clear contrasting density is obtained in the solid image area, a problem occurs in the line image area that the toner bearing amount is relatively excessive. If the toner bearing amount exceeds a predetermined toner bearing amount of the line image area, it is observed that a line image tends to lack sharpness due to a toner flying in a transfer portion. In addition, an input image having the same hue or tone in the solid image area and the line image area may be reproduced as an output image having different hues or tones due to an increase of the toner bearing amount of the line image area. That is, the line image area may become darker than the solid image area.
An image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-345220 determines a line image area and a solid image area in an image frame, and sets the laser light intensity relatively weak in the line image area and relatively strong in the solid image area. With this scheme, the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-345220 obtains an image of good quality in both the line image area and the solid image area in the image frame.
However, when a toner charge amount (charge amount of toner per unit mass) or a developing condition of a developing device is changed, the toner bearing amount on the latent image is also changed. In this case, a ratio of the toner bearing amount of the line image area to the toner bearing amount of the solid image area is changed along with the change in the toner charge amount or the change of the developing condition of the developing device.
Therefore, the hue or tone may not match in the line image area and the solid image area or a phenomenon of the toner flying may occur in the line image area by merely setting the exposure amount of the line image area relatively weaker than the exposure amount of the solid image area with a fixed ratio.